DONETSK, Ukraine — Tensions escalated in eastern Ukraine on Wednesday, with pro-Russian gunmen storming City Hall in the sprawling city of Donetsk and a cluster of Ukrainian troops meant to be restoring order in the region apparently defecting to the side of separatists.

The events signaled a challenge ahead for the pro-Western Ukrainian government on the second day of its campaign to quell the restive east and came as Russian President Vladimir Putin warned that Ukraine was on the verge “of a civil war.”

A line of combat vehicles flying Russian flags rolled Wednesday morning into Slovyansk, a city of 120,000 where separatists have set up roadblocks since Saturday. One soldier named Andrei, speaking to the Associated Press, identified the men as part of the 25th Brigade of Ukraine’s airborne forces that had switched to the side of pro-Russian forces. The troops, in green camouflage and packing automatic weapons and grenade launchers, received a warm welcome from local separatists, AP said.

The report could not be immediately verified, and it was unclear whether the troops, if they did defect, were acting on their own free will.

Around 10 a.m. local time, a squad of separatists backed by seven masked gunmen in camouflage stormed the seat of Donetsk’s mayor and local council.

By Wednesday afternoon, more than 40 men, some masked and heavily armed, were occupying the building but still allowing workers and local officials to go about their business inside city hall.